The present invention relates to a prosthetic spinal disc nucleus. More particularly, it relates to a prosthetic spinal disc nucleus having at least two independent, selectively coupled bodies.
The vertebral spine is the axis of the skeleton upon which all of the body parts "hang". In humans, the normal spine has seven cervical, twelve thoracic and five lumbar segments. The lumbar segments sit upon a sacrum, which then attaches to a pelvis, in turn supported by hip and leg bones. The bony vertebral bodies of the spine are separated by intervertebral discs, which act as joints, but allow known degrees of flexion, extension, lateral bending and axial rotation.
The typical vertebra has a thick interior bone mass called the vertebral body, and a neural (vertebral) arch that arises from a posterior surface of the vertebral body. Each narrow arch combines with the posterior surface of the vertebral body and encloses a vertebral foramen. The vertebral foramina of adjacent vertebrae are aligned to form a vertebral canal, through which the spinal sac, cord and nerve rootlets pass. The portion of the neural arch that extends posteriorly and acts to protect a posterior side of the spinal cord is known as the lamina. Projecting from the posterior region of the neural arch is a spinous process. The central portions of adjacent vertebrae are each supported by an intervertebral disc.
The intervertebral disc primarily serves as a mechanical cushion between the vertebral bones, permitting controlled motions within vertebral segments of the axial skeleton. The normal disc is a unique, mixed structure, comprised of three component tissues: The nucleus pulposus ("nucleus"), the anulus fibrosus ("anulus"), and two opposing vertebral end plates. The two vertebral end plates are each composed of thin cartilage overlying a thin layer of hard, cortical bone which attaches to the spongy, richly vascular, cancellous bone of the vertebral body. The end plates thus serve to attach adjacent vertebrae to the disc. In other words, a transitional zone is created by the end plates between the malleable disc and the bony vertebrae.
The anulus of the disc is a tough, outer fibrous ring that binds together adjacent vertebrae. This fibrous portion, which is much like a laminated automobile tire, is generally about 10 to 15 millimeters in height and about 15 to 20 millimeters in thickness. The fibers of the anulus consist of 15 to 20 overlapping multiple plies, and are inserted into the superior and inferior vertebral bodies at roughly a 30 degree angle in both directions. This configuration particularly resists torsion, as about half of the angulated fibers will tighten when the vertebrae rotate in either direction, relative to each other. The laminated plies are less firmly attached to each other.
Immersed within the anulus, positioned much like the liquid core of a golf ball, is the nucleus. The anulus and opposing end plates maintain a relative position of the nucleus in what can be defined as a nucleus cavity. The healthy nucleus is largely a gel-like substance having a high water content, and similar to air in a tire, serves to keep the anulus tight yet flexible. The nucleus-gel moves slightly within the anulus when force is exerted on the adjacent vertebrae with bending, lifting, etc.
The nucleus and the inner portion of the anulus have no direct blood supply. In fact, the principal nutritional source for the central disc arises from circulation within the opposing vertebral bodies. Microscopic, villous-like fingerlings of the nuclear and anular tissue penetrate the vertebral end plates and allow fluids to pass from the blood across the cell membrane of the fingerlings and then inward to the nuclear tissue. These fluids are primarily body water and the smallest molecular weight nutrients and electrolytes.
The natural physiology of the nucleus promotes these fluids being brought into, and released from, the nucleus by cyclic loading. When fluid is forced out of the nucleus, it passes again through the end plates and then back into the richly vascular vertebral bodies. The cyclic loading amounts to daily variations in applied pressure on the vertebral column (e.g., body weight and muscle pull) causing the nucleus to expel fluids, followed by periods of relaxation and rest, resulting in fluid absorption or swelling by the nucleus. Thus, the nucleus changes volume under loaded and non-loaded conditions. Further, the resulting tightening and loosening effect on the anulus stimulates the normal anulus collagen fibers to remain healthy or to regenerate when torn, a process found in all normal ligaments related to body joints. Notably, the ability of the nucleus to release and imbibe fluids allows the spine to alter its height and flexibility through periods of loading or relaxation. Normal loading cycling is thus an effective nucleus and inner anulus tissue fluid pump, not only bringing in fresh nutrients, but perhaps more importantly, removing the accumulated, potentially autotoxic by-products of metabolism.
The spinal disc may be displaced or damaged due to trauma or a disease process. A disc herniation occurs when the anulus fibers are weakened or torn and the inner tissue of the nucleus becomes permanently bulged, distended, or extruded out of its normal, internal anular confines. The mass of a herniated or "slipped" nucleus can compress a spinal nerve, resulting in leg pain, loss of muscle control, or even paralysis. Alternatively, with discal degeneration, the nucleus loses its water binding ability and deflates, as though the air had been let out of a tire. Subsequently, the height of the nucleus decreases, causing the anulus to buckle in areas where the laminated plies are loosely bonded. As these overlapping laminated plies of the anulus begin to buckle and separate, either circumferential or radial anular tears may occur, which may contribute to persistent and disabling back pain. Adjacent, ancillary spinal facet joints will also be forced into an overriding position, which may create additional back pain.
Whenever the nucleus tissue is herniated or removed by surgery, the disc space will narrow and may lose much of its normal stability. In many cases, to alleviate pain from degenerated or herniated discs, the nucleus is removed and the two adjacent vertebrae surgically fused together. While this treatment alleviates the pain, all discal motion is lost in the fused segment. Ultimately, this procedure places greater stress on the discs adjacent the fused segment as they compensate for the lack of motion, perhaps leading to premature degeneration of those adjacent discs. A more desirable solution entails replacing in part or as a whole the damaged nucleus with a suitable prosthesis having the ability to complement the normal height and motion of the disc while stimulating the natural disc physiology.
The first prostheses embodied a wide variety of ideas, such as ball bearings, springs, metal spikes and other perceived aids. These prosthetic discs were designed to replace the entire intervertebral disc space and were large and rigid. Beyond the questionable efficacy of those devices was the inherent difficulties encountered during implantation. Due to their size and inflexibility, these first generation devices required an anterior implantation approach as the barriers presented by the lamina and, more importantly, the spinal cord and nerve rootlets during posterior implantation, could not be avoided. Recently, smaller and more flexible prosthetic nucleus devices have been developed. With the reduction in prosthesis size, the ability to work around the spinal cord and nerve rootlets during posterior implantation has become possible.
Generally speaking, these reduced size prostheses are intended to serve as a replacement for the natural nucleus. In other words, the anulus and end plates remain intact, and the prosthesis implanted within the nucleus cavity. It is generally believed that this approach facilitates healing of the anulus. To this end, a number of different prosthetic nucleus designs have been developed. A common concern associated with these designs is minimizing damage or stress on the anulus during implantation. In order to implant a prosthesis within the nucleus cavity, an opening or passage must be created through the anulus. Obviously, the smaller the anulus opening required by the particular prosthetic nucleus design, the lesser the damage caused to the anulus. With this in mind, two general design techniques have been identified for reducing the requisite anulus opening size. First, the prosthesis may be configured to increase from a relatively small size prior to implant, to a larger size following implant. With this approach, the prosthesis will have a reduced size prior to implant, thereby minimizing the requisite opening in the anulus. Alternatively, the prosthesis may include several independent, relatively small portions, each of which are implanted through a correspondingly small opening in the anulus.
For example, Bao et al., U.S. Pat. No. 5,047,055 discloses a prosthetic nucleus made of a hydrogel material that is implanted into the intradiscal space in a dehydrated state. Following implant, the hydrogel material hydrates and expands without constraint to, at least in theory, a shape conforming to the natural nucleus. The device of Bao, as well as other similar products, relies solely upon the natural anulus to constrain expansion of the hydrogel core. This essentially uncontrolled expansion imparts a lateral force directly upon the anulus. In most situations, the anulus is already damaged, and any additional forces placed on the anulus by the prosthesis may impede healing and even cause further deterioration. Further, it is virtually impossible to accurately orientate the dehydrated prosthesis of Bao within the nucleus cavity due to the confined environment presented. Finally, although the disclosure of Bao describes a device having a greatly decreased dehydrated size, it stands to reason that an actual product having a hydrated volume equal to a volume of the nucleus cavity would still have a substantial size in the dehydrated state, regardless of the hydrogel material employed.
An alternative prosthetic nucleus design is described in Ray et al., U.S. Pat. No. 5,674,295. Ray describes a hydrogel-based prosthetic nucleus that is implanted into the intradiscal space in a dehydrated state. The Ray et al. prosthesis includes a jacket sized to constrain expansion of the hydrogel core. More particularly, following implant, the constraining jacket directs the hydrogel core to expand primarily in height, thereby separating adjacent vertebrae. The prosthetic spinal disc nucleus of Ray et al. is sized such that in a final hydrated form, the prosthesis has a volume much less than a volume of the nucleus cavity. In this way, two prostheses can be orientated in a side-by-side fashion within the nucleus cavity. With this dual-prosthesis approach, only a small incision in the anulus is required for implantation. The prostheses are implanted through the small opening, one after the other. Other prostheses, while not being hydrogel-based, similarly follow this dual or multi-component approach.
While the device of Ray, along with other variations, are clearly beneficial, certain concerns may arise. In particular, while the multi-component prosthesis undoubtedly facilitates use of a small anulus opening, because each of the individual components are correspondingly small, there is a possibility that one or more of the components will extrude or eject back through the anulus opening. In other words, each component has a size generally corresponding to a size of the anulus opening. Even if this opening is sewn shut following implant, various forces acting upon the spine may have the potential to "push" one or more of the components back through the anulus opening. Some efforts have been made to address this problem, such as providing the prosthesis component with an expandable tine assembly. Stubstad et al., U.S. Pat. No. 3,867,728 mentions tying two prosthesis segments together with a cord following implant. Unfortunately, due to the highly confined nature of the nucleus cavity, it would be virtually impossible for a surgeon to manipulate the cord extending from one segment around the second segment. In other words, because the cord is not in any way connected to the second segment, the surgeon must establish this connection post-implant. The anulus and opposing end plates render this task highly difficult.
Degenerated, painfully disabling intraspinal discs are a major economic and social problem for patients, their families, employers and the public at large. Any significant means to correct these conditions without further destruction or fusion of the disc may therefore serve an important role. Other means to replace the function of a degenerated disc have major problems such as complex surgical procedures, unproven efficacy, placing unnecessary and possibly destructive forces on an already damaged anulus, etc. Further, unexpected expulsion of the prosthesis, or individual components, from the disc space following implant while uncommon, may be a potential concern. Therefore, a substantial need exists for a prosthetic spinal disc nucleus configured to minimize damage to the anulus and reduce the potential for expulsion following implant.